Woolly adelgids were first observed in in this area in 2000, elongated hemlock scale in 2006 and a blight called sirococcus tsugae last year.

The spread northward is being aided by climate change, Weiner said.

Those pests, plus the last couple of years of drought, have stressed the trees, she said. They could probably survive any one of them, but when you get two or three, it weakens the trees and attracts the native hemlock borers.

Woodpeckers like the borer larvae, she said, and dig them out, leaving holes. She showed a picture of a tree trunk covered with holes caused mainly by the birds.

But Weiner said there are things a person can do to save the trees that are important to you – landscape trees, and especially nice or old ones.

“We do an annual survey for adelgid,” she said, “checking the northern border. We have found all of these in Lyndeborough.”

Weiner was accompanied by Lee Gilman, of Amherst, a professional arborist, who described the various kinds of systemic treatments. Some are applied to the soil around the trunk, and some are sprayed on the branches.

“There are many nontoxic kinds,” he said, “which work well, but take a little longer.”

He also noted, “You can’t treat the whole forest.” A stand of trees should be thinned to the best, and those should then be cared for.

Weiner listed the various pests, noting what should be looked for.

For adelgids, she said, “Look for little fuzzy balls, where they get their name ‘woolly.’ “ For scale, “Look on the needles.” The fungus causes the branch tips to die.

Asked about the future of the trees, she said, “Within the next 10 years, I expect a lot of mortality. In the long term, we don’t know. Who knows what we will come up with?”

Resident Leo Trudeau asked about the effects of pesticides on soil and water.

Where the ground is treated, right around the tree, it kills the worms, Gilman said, but it “isn’t particularly mobile in the soil.” He added, “An integrated pest management program is the best.”

Trudeau said he was removing trees as they died.

Weiner was the library’s Third Monday speaker, a program arranged by program coordinator Regina Conrad.